Project Summary/Abstract The normal remodeling of bone requires synchronized activity between bone resorbing osteoclastic and bone forming osteoblastic cells. This synchronization includes the recruitment of osteoblasts and osteoclasts, and the stimulation of osteoclast and osteoblast proliferation and differentiation. Imbalances between bone resorption and formation may lead to bone disorders, such as osteoporosis and osteopetrosis. Gap junctions (GJ) are membrane-spanning channels that allow passage of ions and signaling molecules, less than 1KD in size, between two adjacent cells. Each gap junction is composed of two hemichannels or connexons and each connexon is comprised of 6 connexins. The predominant form of connexin in bone cells is Cx43. Previous In vitro studies demonstrated that Cx43, hemichannel and GJ regulate osteoblast, osteocyte and osteoclast differentiation and function. However, little is known about the in vivo function of Cx43 and GJIC in bone. To address this deficit, we specifically deleted Cx43 in mouse osteoblasts and osteocytes. The knockout mice displayed osteopenic phenotypes with increased osteoclast number. Therefore, in this application, we hypothesize that osteoblastic lineage cells, including osteoblasts and osteocytes, communicate with osteoclastic cells and inhibit osteoclastogenesis via Cx43 and GJIC. Our hypothesis predicts that reducing Cx43 levels and heterotypic GJIC between osteoblastic lineage cells and osteoclastic lineage cells leads to increased osteoclastic activity. We will examine whether osteoblastic lineage cells regulate osteoclastogenesis via Cx43 and GJIC by completing three aims over a 3 year period: Aim 1, demonstrate Cx43-mediated GJIC between osteoblastic or osteocytic and osteoclastic cells;Aim 2, quantify osteoclast cell number and activity, both in vivo and in vitro, induced by osteoblasts and osteocytes with different Cx43 levels;and aim 3, investigate the mechanism of Cx43 and GJIC mediated osteoblastic and osteocytic cell regulated osteoclastogenesis. By completing these aims we will not only identify new functions of Cx43 and GJIC in bone, but will also identify novel mechanisms contributing to osteopenia. PUBLIC HEALTH RELEVANCE: Project Narrative We will examine whether osteoclasts communicate with osteocblastic lineage cells via Cx43 mediated gap junctional intercellular communication (GJIC). Furthermore, We will also investigate the role and mechanism of Cx43 and GJIC in osteoclastogenesis regulated by osteoblastic lineage cells. The completion of the proposed aims will greatly help to develop novel therapeutic targets for disorders of bone metabolism including osteopenias and osteopetrosis.